The present invention relates to processes for producing 2-alkyl-2-cyclopentenones useful as industrial products in organic synthetic chemistry, such as medicaments and flavor and fragrance raw materials, and also intermediates thereof.
Heretofore, 2-alkyl-2-cyclopentenones represented by the general formula (2) are known as fruity and floral synthetic fragrance raw materials including jasmine as a representative. In particular, 2-amyl-2-cyclopentenone has a jasmine-like fruity and floral odor and is useful as a fragrance raw material [e.g., Gosei Koryo (Synthetic Flavor and Fragrance Raw Materials)-Kagaku to Shohin Chishiki (Chemistry and Knowledge on Commercial Products)xe2x80x94(written by Motoichi Indo, Kagaku Kogyo Nippo) p. 353].
Moreover, 2-alkyl-2-cyclopentenones are important as synthetic intermediates. For example, as described in Gosei Koryo (Synthetic Flavor and Fragrance Raw Materials)xe2x80x94Kagaku to Shohin Chishiki (Chemistry and Knowledge on Commercial Products)xe2x80x94(written by Motoichi Indo, Kagaku Kogyo Nippo) pp. 843-847, methyl jasmonate and methyl dihydrojasmonate are obtained by adding methyl malonate to 2-cis-pentenyl- and 2-pentyl-2-cyclopentenone and then subjecting the addition products to decarboxylation, respectively. These jasmonates are useful as flavor and fragrance raw materials having a jasmine-like floral odor.
Furthermore, 2-alkylcyclopentanones obtainable by hydrogenating the double bond of 2-alkyl-2-cyclopentenones are useful as flavor and fragrance raw materials. For example, it is described in Gosei Koryo (Synthetic Flavor and Fragrance Raw Materials)-Kagaku to Shohin Chishiki (Chemistry and Knowledge on Commercial Products)xe2x80x94(written by Motoichi Indo, Kagaku Kogyo Nippo) pp. 353-358 that 2-pentylcyclopentanone, 2-hexylcyclopentanone and 2-heptylcyclopentanone are useful as jasmine-like fruity and floral synthetic flavor and fragrance raw materials.
In addition, xcex4-lactones obtainable by oxidizing 2-alkylcyclopentanones, for example, using peracetic acid are useful as flavor and fragrance raw materials [Gosei Koryo (Synthetic Flavor and Fragrance Raw Materials)-Kagaku to Shohin Chishiki (Chemistry and Knowledge on Commercial Products)xe2x80x94(written by Motoichi Indo, Kagaku Kogyo Nippo) pp. 562-568].
Heretofore, as a process for producing a 2-alkyl-2-cyclopentenone represented by the formula (2), a process represented by the following reaction scheme (1) has been widely used. 
The process involves the following three steps:
First step: a step of obtaining a 2-(1-hydroxyalkyl)cyclopentanone represented by the general formula (1) by treating a cyclopentanone represented by the general formula (4) with an aliphatic carbonyl compound represented by the general formula (5);
Second step: a step of obtaining a 2-alkylidenecyclopentanone represented by the general formula (3) by dehydrating a 2-(1-hydroxyalkyl)cyclopentanone represented by the general formula (1); and
Third step: a step of producing a 2-alkyl-2-cyclopentenone represented by the general formula (2) by isomerizing a 2-alkylidenecyclopentanone represented by the general formula (3).
Of these steps, the first step is an application of the reaction, so-called aldol reaction which is well known and, in the case of existing no particular hindrance such as steric hindrance, a 2-(1-hydroxyalkyl)cyclopentanone is obtained in good yields under relatively mild conditions. Moreover, a process is also known wherein a 2-(1-hydroxyalkyl)cyclopentanone, a product in the first step is simultaneously dehydrated to afford a 2-alkylidenecyclopentanone in only one step. In this case, the reaction is particularly called as aldol condensation reaction.
However, there is room for improvement in the above production of a 2-alkyl-2-cyclopentenone via the above reaction or aldol condensation in view of the yields and operational methods and hence the process is by no means industrially advantageous.
For example, as shown in the following reaction scheme (2), U.S. Pat. No. 4,310,701 discloses a process for obtaining a 2-alkyl-2-cyclopentenone (9) by dehydrating an aldol (7) obtainable from the aldol reaction of cyclopentanone (6) with an aliphatic aldehyde using oxalic acid as a catalyst to afford a 2-alkylidenecyclopentanone (8) and heating it together with an n-butanol solution of concentrated hydrochloric acid to isomerize the double bond. 
However, in this process, the yield of the compound (9) calculated based on the compound (6) is not so high and hence the process is not satisfactory. Moreover, some problems, e.g., the inevitable use of a large excess of n-butanol and a large amount of concentrated hydrochloric acid in the isomerization reaction from the compound (8) to the compound (9), are pointed out in view of the production efficiency and thus there is room for improvement as an industrial production process.
Furthermore, JP-A-5-92934 discloses a process for synthesizing a 2-alkyl-2-cyclopentenone (9) by subjecting the aldol (7) to dehydration and isomerization simultaneously in concentrated hydrochloric acid-n-butanol. However, even in this process, the yield of the compound (9) is not satisfactory and productivity is poor because of the use of a large excess of n-butanol and a large amount of concentrated hydrochloric acid. Thus, for the practical use, there is room for improvement.
Additionally, as a catalyst for the reaction of isomerizing the 2-alkylidenecyclopentanone (8) to the 2-alkyl-2-cyclopentenone (9), transition metal complex catalysts are known (JP-B-58-42175 and JP-B-59-29051). However, this process has problems that it produces relatively a large amount of by-products and these catalysts are expensive, so that it is desired to develop an isomerization catalyst which is inexpensive and also excellent in selectivity.
Moreover, as disclosed in JP-A-2000-327618, JP-A-59-80625 and JP-A-51-23240, isomerization processes using a hydrogen halide or a sulfonic acid are also known. However, in these cases, a large amount of a solvent is used and there is room for improvement of the yield, so that the processes are not industrially advantageous.
An object of the invention is to provide industrially advantageous production processes including production of a 2-alkyl-2-cyclopentenone in high yields using a 2-(1-hydroxyalkyl)cyclopentanone or a 2-alkylidenecyclopentanone, which are obtainable from a cyclopentanone and a carbonyl compound.
As a result of intensive studies in consideration of the above circumstances, the present inventors have found that dehydration and isomerization occur simultaneously in high selectivity by reacting a 2-(1-hydroxyalkyl)cyclopentanone represented by the general formula (1) in the presence of a bromine compound and/or an iodine compound as shown in the following reaction scheme (3), and thereby a 2-alkyl-2-cyclopentenone represented by the general formula (2) is obtained in high yields. 
Furthermore, as shown in the following reaction scheme (4), they have found that the exocyclic double bond is selectively isomerized to endcyclic double bond by reacting a 2-alkylidenecyclopentanone represented by the following formula (3) in the presence of a catalytic amount of a bromine compound and/or an iodine compound, and thereby a 2-alkyl-2-cyclopentenone represented by the general formula (2) is obtained in high yields. 
They have accomplished the invention as a result of further more studies.
That is, the invention relates to the following.
1) A process for producing a 2-alkyl-2-cyclopentenone represented by the following general formula (2): 
(wherein R1, R2, R3, R4, R5, R6 and R7 each independently represents hydrogen atom, an alkyl group having 1 to 10 carbon atoms which may have one or more substituents or an aromatic group which may have one or more substituents, and each of (1) R6 or R7 with R3 and (2) R6 or R7 with R4 or R5 may be together combined to form a ring which may have a double bond),
which comprises a step of dehydrative isomerization of a 2-(1-hydroxyalkyl)cyclopentanone represented by the following general formula (1): 
(wherein R1, R2, R3, R4, R5, R6 and R7 are the same as above) in the presence of a bromine compound and/or an iodine compound.
2) A process for producing a 2-alkyl-2-cyclopentenone represented by the above general formula (2), which comprises a step of isomerization of a 2-alkylidenecyclopentanone represented by the following general formula (3): 
(wherein R1, R2, R3, R4, R5, R6 and R7 are the same as above) in the presence of a bromine compound and/or an iodine compound.